Antilock brake control system for vehicle

ABSTRACT

An antilock brake control system for a vehicle includes a PID calculating device for carrying out a PID calculation based on a deviation between a target slip rate determined based on a presumed vehicle speed and a slip rate, and the operation of a braking liquid pressure regulating device is controlled based on the result of the calculation in the PID calculating device. The PID calculating device is arranged to limit the PID calculation value within a predetermined limit value. Thus, it is possible to enhance the control followability in carrying out the control of a braking pressure based on the PID calculation.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an antilock brake control systemincluding wheel speed detecting means for detecting a wheel speed, avehicle speed presuming means for detecting a vehicle speed based on thewheel speed detected by the wheel speed detecting means, a slip ratecalculating means for calculating a slip rate based on the wheel speeddetected by the wheel speed detecting means and the presumed vehiclespeed determined in the vehicle speed presuming means, a target sliprate determining means for determining a target slip rate based on thepresumed vehicle speed determined in the vehicle speed presuming means,a deviation calculating means for calculating a deviation between thetarget slip rate determined in the target slip rate determining meansand the slip rate calculated in the slip rate calculating means, and aPID calculating means for carrying out a PID calculation based on thedeviation determined in the deviation calculating means, wherein theoperation of braking liquid pressure regulating means is controlledbased on the result of a calculation in the PID calculating means.

2. Description of the Related Art

Such a control system is conventionally known, for example, fromJapanese Patent Application Laid-open Nos.6-144195 and 7-117653.

In the above known control system in which the control of a brakingpressure is carried out based on a PID calculation, however, there is apossibility that the PID calculation value may become too large due toan alteration of data caused by an unexpected phenomenon such as thegeneration of a noise around a calculating system. If the control of thebraking pressure is continued based on such excessively large PIDcalculation value, there is a possibility that a difference between thewheel-slipping state presumed in the calculation system and an actualwheel- slipping state may be increased, and when a sudden variation isthereafter produced in the wheel slipping state due to a variation infriction coefficient of a road surface or the like, a delay may beproduced in the control of the braking pressure, resulting in a degradedcontrol followability.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide anantilock brake control system for a vehicle, wherein the controlfollowability is enhanced in carrying out the control of the brakingpressure based on the PID calculation.

To achieve the above object, according to a first aspect of the presentinvention, there is provided an antilock brake control system for avehicle, comprising wheel speed detecting means for detecting a wheelspeed, a vehicle speed presuming means for determining a vehicle speedbased on the wheel speed detected by the wheel speed detecting means, aslip rate calculating means for calculating a slip rate based on thewheel speed detected by the wheel speed detecting means and the presumedvehicle speed determined in the vehicle speed presuming means, a targetslip rate determining means for determining a target slip rate based onthe presumed vehicle speed determined in the vehicle speed presumingmeans, a deviation calculating means for calculating a deviation betweenthe target slip rate determined in the target slip rate determiningmeans and the slip rate calculated in the slip rate calculating means,and a PID calculating means for carrying out a PID calculation based onthe deviation determined in the deviation calculating means, wherein theoperation of braking liquid pressure regulating means is controlledbased on the result of a calculation in the PID calculating means, andwherein the PID calculating means is arranged to limit a PID calculationvalue within a predetermined limit value.

According to a second aspect of the present invention, there is providedan antilock brake control system for a vehicle, comprising wheel speeddetecting means for detecting a wheel speed, a vehicle speed determiningmeans for presuming a vehicle speed based on the wheel speed detected bythe wheel speed detecting means, a slip rate calculating means forcalculating a slip rate based on the wheel speed detected by the wheelspeed detecting means and the presumed vehicle speed determined in thevehicle speed presuming means, a target slip rate determining means fordetermining a target slip rate based on the presumed vehicle speeddetermined in the vehicle speed presuming means, a deviation calculatingmeans for calculating a deviation between the target slip ratedetermined in the target slip rate determining means and the slip ratecalculated in the slip rate calculating means, and a PID calculatingmeans for carrying out a PID calculation based on the deviationdetermined in the deviation calculating means, wherein an operation ofbraking liquid pressure regulating means is controlled based on theresult of a calculation in the PID calculating means, and wherein thePID calculating means is arranged to limit calculation values providedby a proportional operation, an integrating operation and adifferentiating operation to mutually independent predetermined limitvalues, respectively.

Further, according to a third aspect of the present invention, there isprovided an antilock brake control system for a vehicle, comprisingwheel speed detecting means for detecting a wheel speed, a vehicle speedpresuming means for determining a vehicle speed based on the wheel speeddetected by the wheel speed detecting means, a slip rate calculatingmeans for calculating a slip rate based on the wheel speed detected bythe wheel speed detecting means and the presumed vehicle speeddetermined in the vehicle speed presuming means, a target slip ratedetermining means for determining a target slip rate based on thepresumed vehicle speed determined in the vehicle speed presuming means,a deviation calculating means for calculating a deviation between thetarget slip rate determined in the target slip rate determining meansand the slip rate calculated in the slip rate calculating means, and aPID calculating means for carrying out a PID calculation based on thedeviation determined in the deviation calculating means, wherein anoperation of braking liquid pressure regulating means is controlledbased on the result of a calculation in the PID calculating means, andwherein the PID calculating means is arranged to limit a differencebetween a last PID calculation value and a current PID calculation valueto a predetermined limit value.

With the first aspect of the present invention, the PID calculatingmeans is arranged to limit the PID calculation value to be not more thanthe predetermined limit value. With the second aspect of the presentinvention, the PID calculating means is arranged to limit thecalculation values provided by the proportional, integrating anddifferentiating operations to the mutually independent limit values,respectively. With the third aspect of the present invention, the PIDcalculating means is arranged to limit the difference between the lastPID calculation value and a current PID calculation value to thepredetermined limit value. With these arrangements, even if analteration of data is occurred due to the unexpected phenomenon such asthe generation of a noise or the like, the PID calculation value can beprevented from becoming too large. Even when a sudden variation isproduced in the wheel slipping state due to a variation in frictioncoefficient of a road surface, a delay in the control of the brakingpressure can be avoided to enhance the control followability.

According to a preferred feature of the present invention, in additionto the first, second or third aspect, the antilock brake control systemfurther includes a friction coefficient determining means fordetermining a friction coefficient of a road surface, a suspensionvibration determining means for determining a vibrated state of asuspension, a limit value determining means for determining the limitvalue in the PID calculating means based on the presumed vehicle speeddetermined in the vehicle speed presuming means, a result of adetermination in the friction coefficient determining means and a resultof a determination in the suspension vibration determining means. Thus,the limit values in the PID calculating means are determined inaccordance with the friction coefficient of the road surface and thevibrated state of the suspension. Therefore, it is possible to setappropriate limit values for the traveling situation of the vehicle.

The above and other objects, features and advantages of the inventionwill become apparent from the following description of the preferredembodiment taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of the entire arrangement of a brake system ina motorcycle;

FIG. 2 is a block diagram illustrating the arrangement of a controlunit;

FIG. 3 is a flow chart illustrating a vehicle speed presuming procedure;

FIG. 4 is a diagram for explaining a process for calculating a presumedvehicle speed based on a wheel speed;

FIG. 5 is a block diagram illustrating the arrangement of a front wheelcontrol section;

FIG. 6 is a diagram illustrating a map for deciding a control mode;

FIGS. 7A through 7E are timing charts for explaining the selection of acontrol mode;

FIGS. 8A through 8D are a timing charts for explaining the change of thecontrol mode;

FIGS. 9A and 9B are diagrams illustrating a timing chart when a PIDcalculation value has been limited by a limit value, in contrast withthat when the calculation value has not been limited;

FIGS. 10A and 10B are diagrams illustrating a timing chart when acalculation value provided by a proportional operation has been limitedby a limit value, in contrast with that when the calculation value hasnot been limited;

FIGS. 11A and 11B are diagrams illustrating a timing chart when acalculation value provided by an integrating operation has been limitedby a limit value, in contrast with that when the calculation value hasnot been limited;

FIGS. 12A and 12B are diagrams illustrating a timing chart when acalculation value provided by a differentiating operation has beenlimited by a limit value, in contrast with that when the calculationvalue has not been limited; and

FIGS. 13A through 13E are diagrams illustrating a timing chart when thePID calculation value has been filtered in a low-pass filter, incontrast with that when the PID calculation value has not been filtered.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention will now be described by way of an embodimentapplied to a motorcycle with reference to FIGS. 1 to 13.

Referring first to FIG. 1, a braking liquid pressure regulating means3_(F) is provided between a master cylinder 2 adapted to output a liquidpressure depending upon the operation of a brake lever 1 and a pair ofleft and right front wheel brakes B_(F1) and B_(F2) mounted to a frontwheel of a motorcycle. The braking liquid pressure regulating means3_(F) is capable of regulating braking liquid pressures for both thefront wheel brakes B_(F1) and B_(F2). A rear wheel braking liquidpressure regulating means 3_(R) is provided between a master cylinder 5adapted to output a liquid pressure depending upon the operation of abrake pedal 4 and a rear wheel brake B_(R) mounted on a rear wheel ofthe motorcycle, and is capable of regulating a braking liquid pressurefor the rear wheel brake B_(R).

The braking liquid pressure regulating means 3_(F) includes a reservoir6_(F), a normally-opened solenoid valve 7_(F) mounted between both thefront wheel brakes B_(F1) and B_(F2) and the master cylinder 2, anormally-closed solenoid valve 8_(F) mounted between the reservoir 6_(F)and both the front wheel brakes B_(F1) and B_(F2), a check valve 9_(F)connected parallel to the normally-opened solenoid valve 7_(F) to permita braking liquid to flow from both the front wheel brakes B_(F1) andB_(F2) toward the master cylinder 2, and a return pump 11_(F) having aninlet connected to the reservoir 6_(F) through an intake valve 10_(F)and an outlet connected to the master cylinder 2 through a dischargevalve 12_(F).

The braking liquid pressure regulating means 3_(R) is constructed in thesame manner as is the braking liquid pressure regulating means 3_(F1)and includes a reservoir 6_(R), a normally-opened solenoid valve 7_(R),a normally-closed solenoid valve 8_(R), a check valve 9_(R), an intakevalve 10_(R), a return pump 11_(R), and a discharge valve 12_(R).

Moreover, the return pump 11_(F) of the braking liquid pressureregulating means 3_(F) and the return pump 11_(R) of the braking liquidpressure regulating means 3_(R) are driven by a common motor 13.

Controlled by a control unit 14 are the normally-opened and closedsolenoid valves 7_(F) and 8_(F) of the braking liquid pressureregulating means 3_(F), the normally-opened and closed solenoid valves7_(R) and 8_(R) of the braking liquid pressure regulating means 3_(R)and the motor 13. Inputted to the control unit 14 are output signalsfrom a front wheel speed sensor 16_(F) fixedly disposed on an opposedrelation to a side of a pulser gear 15_(F) fixed to the front wheel, arear wheel speed sensor 16_(R) fixedly disposed on an opposed relationto a side of a pulser gear 15_(R) fixed to the rear wheel, a brakeswitch 17_(F) for the front wheel brakes and a brake switch 17_(R) forthe rear brake. The control unit 14 controls the operations of thenormally-opened solenoid valves 7_(F) and 7_(R), the normally-closedsolenoid valves 8_(F) and 8_(R) and the motor 13 in accordance with theoutputs from the sensors 16_(F) and 16_(R) and the switches 17_(F) and17_(R).

The arrangement of those sections in the control unit 14 which areconcerned with the antilock brake control will be described below withreference to FIG. 2. The control unit 14 includes a front wheel speedcalculating means 20_(F), a front wheel acceleration or decelerationcalculating means 21_(F), a front wheel-use vehicle speed calculatingmeans 22_(F), and a front wheel-side control section 23_(F), all incorrespondence to the braking liquid pressure regulating means 3_(F) forthe front wheel. The control unit 14 also includes a rear wheel speedcalculating means 20_(R), a rear wheel acceleration or decelerationcalculating means 21_(R), a rear wheel-use vehicle speed calculatingmeans 22_(R), and a rear wheel-side control section 23_(R), all incorrespondence to the braking liquid pressure regulating means 3_(R) forthe rear wheel. Further, the control unit 14 includes a referencevehicle speed determining means 24 as a vehicle speed presuming meanscommon to both the braking liquid pressure regulating means 3_(F) and3_(R).

A control quantity determined in the front wheel-side control section23_(F) is inputted to a solenoid driving means 25_(F) for the frontwheel. The normally-opened solenoid valve 7_(F) and the normally-closedsolenoid valve 8_(F) in the front wheel braking liquid pressureregulating means 3_(F) are opened and closed by the solenoid drivingmeans 25_(F) for the front wheel. In addition, a control quantitydetermined in the rear wheel-side control section 23_(R) is inputted toa solenoid driving means 25_(R) for the rear wheel. The normally-openedsolenoid valve 7_(R) and the normally-closed solenoid valve 8_(R) in therear wheel braking liquid pressure regulating means 3_(R) are opened andclosed by the solenoid driving means 25_(R) for the rear wheel. Further,the motor 13 common to both the braking liquid pressure regulating means3_(F) and 3_(R) is operated by a motor driving means 26 in response tothe application of the control quantities for carrying out the antilockbrake control from the front and rear wheel-side control sections 23_(F)and 23_(R) to the motor driving means 26.

The front wheel speed calculating means 20_(F) calculates a front wheelspeed under reception of the output signal from the front wheel speedsensor 16_(F), and constitutes a front wheel speed detecting means19_(F) together with the front wheel speed sensor 16_(F). The rear wheelspeed calculating means 20_(R) calculates a rear wheel speed underreception of the output signal from the rear wheel speed sensor 16_(R),and constitutes a rear wheel speed detecting means 19_(R) together withthe rear wheel speed sensor 16_(R).

Both the wheel acceleration or deceleration calculating means 21_(F) and21_(R), both the vehicle speed calculating means 22_(F) and 22_(R), aswell as the front and rear wheel-side control sections 23_(F) and 23_(R)have the same function, respectively. Therefore, only the front wheelacceleration or deceleration calculating means 21_(F), the frontwheel-use vehicle speed calculating means 22_(F) and the frontwheel-side control section 23_(F) will be described, and the descriptionof the rear wheel acceleration or deceleration calculating means 21_(R),the rear wheel-use vehicle speed calculating means 22_(R) and the rearwheel-side control section 23_(R) are omitted.

The wheel front acceleration or deceleration calculating means 21_(F)differentiates the front wheel speed calculated in the front wheel speedcalculating means 20_(F) in the front wheel speed detecting means 19_(F)to provide a front wheel acceleration or deceleration.

The front wheel-use vehicle speed calculating means 22_(F) calculates apresumed front vehicle speed based on the front wheel speed detected bythe front wheel speed detecting means 19_(F), and the front wheelacceleration or deceleration calculated in the front wheel accelerationor deceleration calculating means 21_(F). The presumed vehicle speed iscalculated according to a procedure shown in FIG. 3.

At step S1 in FIG. 3, the front wheel speed VW detected by the frontwheel speed detecting means 19_(F) and the front wheel acceleration ordeceleration dVW calculated in the front wheel acceleration ordeceleration calculating means 21_(F) are read. At step S2, it isdetermined whether a flag F is equal to "0". If F=0 at step S3, thefront wheel speed VW is brought into a presumed vehicle speed VR andthen, at step S4, the flag F is set at "1". These steps S1 to S4 areprocessing steps at the start of the calculation of the presumed vehiclespeed. In a next calculating cycle, F=1 and hence, the processing isadvanced from step S2 to step S5.

At step S5, it is determined whether the current wheel speed VW(n) isequal to or smaller than the last presumed vehicle speed VR(n-1), i.e.,whether the front wheel speed is in a constant or reducing course. If itis determined that the front wheel speed is in the constant or reducingcourse, the processing is advanced to step S6, at which it is determinedwhether dVW≦α1, i.e., the deceleration of the front wheel is equal to orlarger than a preset deceleration α1 (e.g., -1G). If VW(n)≦VR(n-1),i.e., if it is determined that the front wheel speed is in the constantor reducing course, the processing is advanced to step S6, at which aflag Fα2 is set at "0". At next step S7, it is determined whether Fα1=1.This flag Fα1 becomes "1" when the acceleration or deceleration is setat the preset deceleration α1 in the reducing course. In a firstprocessing cycle, Fα1=0 and hence, the processing is advanced from stepS7 to step S8.

At step S8, it is determined that dVW≦α1, i.e., the deceleration of thefront wheel is a deceleration equal to or larger than the presetdeceleration α1. If dVW≦α1, the acceleration or deceleration α is set atthe preset deceleration α1 at step S9, and the flag Fα1 is set at "1" atstep S10, progressing to step S11.

At step S11, the calculation of the presumed vehicle speed VR is carriedout. If the last presumed vehicle speed is represented by VR(n-1) andthe time of the calculating cycle is represented by ΔT (e.g., 3 msec),the current presumed vehicle speed VR(n) is calculated according to thefollowing equation:

    VR(n)=VR(n-1)+α·ΔT

If it is determined at step S8 that dVW>α1, the acceleration ordeceleration α is determined at a front acceleration or deceleration dVWat step S12, progressing to step S11. If it is also determined at stepS7 that Fα1=1, the processing is advanced from step S7 to step S11. Inother words, if the front wheel acceleration or deceleration dVW becomesa deceleration equal to or larger than the preset deceleration α1 in thereducing course of the front wheel speed, the calculation of thepresumed vehicle speed VR is carried out on the assumption that thevehicle speed is being decreased at the preset deceleration α1 in asubsequent reducing course.

If it is determined at step S5 that VW(n)>VR(n-1), i.e., the front wheelspeed is in an increasing course, the processing is advanced from stepS5 to step S13, at which the flag Fα1 is set at 0. Then, it isdetermined at step S14 whether a flag Fα2 is equal to 1. This flag Fα2is "1+ when the acceleration or deceleration is set at a presetacceleration α2 in the increasing course. In a first processing cycle ofthe increasing course, Fα2=0 and hence, the processing is advanced fromstep S14 to step S15. At step S15, it is determined whether dVW≧α2,i.e., the acceleration of the front wheel speed is equal to or largerthan the preset acceleration α2. If dVW≧α2, the acceleration ordeceleration α is set at the preset acceleration α2 at step S16, and theflag Fα2 is set at "1" at step S17, progressing to step S11. If dVW<α2,the acceleration or deceleration α is determined at the front wheelacceleration or deceleration dVW at step S18, progressing to step S11.If it is determined at step S14 that Fα2=1, the processing is advanceddirectly to step S15. In other words, if the front wheel acceleration ordeceleration dVW becomes equal to or larger than the preset accelerationα2 in the front wheel speed increasing course, the calculation of thepreset vehicle speed VR is carried out on the assumption that thevehicle speed is being increased at the preset acceleration α2 in asubsequent increasing course.

According to such calculation in the front wheel-use vehicle speedcalculating means 22_(F), the presumed vehicle speed is as shown in FIG.4. In the front wheel reducing course, the calculation of the presumedvehicle speed VR using the deceleration of the front wheel speed iscarried out in such a manner that the presumed vehicle speed does notbecome a deceleration equal to or larger than the preset decelerationα1. In the front wheel speed increasing course, the calculation of thepresumed vehicle speed using the acceleration of the front wheel speedis carried out in such a manner the presumed vehicle speed does notbecome an acceleration equal to or larger than the preset accelerationα2.

The preset acceleration α2 is, for example, +1 G, but may be set alarger value during the antilock brake control, or may be varied inaccordance with the vehicle deceleration.

The reference vehicle speed determining means 24 determines a presumedvehicle speed which is a criterion for determining slip rates of thefront and rear wheels, based on the front wheel-use presumed vehiclespeed calculated in the front wheel-use vehicle speed calculating means22_(F) and the rear wheel-use presumed vehicle speed calculated in therear wheel-use vehicle speed calculating means 22_(R). For example, ahigh select value of the front wheel-use presumed vehicle speedcalculated in the front wheel-use vehicle speed calculating means 22_(F)and the rear wheel-use presumed vehicle speed calculated in the rearwheel-use vehicle speed calculating means 22_(R) is determined as areference presumed vehicle speed.

The front wheel-side control section 23_(F) determines braking liquidpressure control quantities for the front wheel brakes B_(F1) andB_(F2), based on the front wheel speed detected by the front wheel speeddetecting means 19_(F), the front wheel acceleration or decelerationcalculated in the front wheel acceleration or deceleration calculatingmeans 21_(F) and the presumed vehicle speed determined in the referencevehicle speed determining means 24. The front wheel-side control section23_(F) is constructed as shown in FIG. 5.

Referring to FIG. 5, the front wheel-side control section 23_(F)includes a slip rate calculating means 28, a target slip ratedetermining means 29, a deviation calculating means 30, a PIDcalculating means 31, a gain constant setting means 32, a control modedeciding means 33, a suspension vibration determining means 34, afriction coefficient determining means 35, a limit value setting means36 and a passage area setting means 37.

The slip rate calculating means 28 calculates a slip rate of the frontwheel based on the wheel speed detected by the front wheel speeddetecting means 19_(F) and the presumed vehicle speed determined in thereference vehicle speed determining means 24. That is, if the slip rateis represented by SR; the presumed vehicle speed is represented by VR;and the front wheel speed is represented by VW, the slip rate SR iscalculated by the slip rate calculating means 28 according to thefollowing equation:

    SR=(VR-VW)/VR

In the target slip rate determining means 29, the slip rate as a targetduring traveling of the vehicle at the presumed vehicle speed is set asa target slip rate SRobj, based on the presumed vehicle speed determinedin the reference vehicle speed determining means 24. In the deviationcalculating means 30, a deviation ΔS (=SRobj-SR) between the target sliprate SRobj determined in the target slip rate determining means 29 andthe slip rate SR calculated in the slip rate calculating means 28 iscalculated.

The presumed vehicle speed determined in the reference vehicle speeddetermining means 24 and the front wheel speed detected by the frontwheel speed detecting means 19_(F) are inputted to the suspensionvibration determining means 34. The suspension vibration determiningmeans 34 determines a vibrated state of a suspension based on thepresumed vehicle speed and the front wheel speed.

The presumed vehicle speed determined in the reference vehicle speeddetermining means 24 and the front wheel speed detected by the frontwheel speed detecting means 19_(F) are inputted to the frictioncoefficient determining means 35. The friction coefficient determiningmeans 35 determines a friction coefficient of a road surface bycomparison of the vehicle deceleration calculated based on the presumedvehicle speed and the front wheel speed with a preset value.

The presumed vehicle speed determined in the reference vehicle speeddetermining means 24, the front wheel acceleration or decelerationcalculated in the front wheel acceleration or deceleration calculatingmeans 21_(F) and a result of the determination of the frictioncoefficient in the friction coefficient determining means 35 areinputted to the gain constant setting means 32. The gain constantsetting means 32 sets a proportional calculation gain constant Kp, anintegration calculation gain constant Ki and a differentiationcalculation gain constant Kd in a PID calculation in the PID calculatingmeans 31. These gain constants Kp, Ki and Kd are each set in the form ofa function of the presumed vehicle speed determined in the referencevehicle speed determining means 24 in the following manner:

Kp=fp(VR)

Ki=fi(VR)

Kd=fd(VR)

Moreover, the gain constant setting means 32 is arranged to change thefunctions fp(VR), fi(VR) and fd(VR) determining the gain constants Kp,Ki and Kd in accordance with the front wheel acceleration ordeceleration calculated in the front wheel acceleration or decelerationcalculating means 21_(F) and a result of the determination of thefriction coefficient in the friction coefficient determining means 35.

The PID calculating means 31 performs a PID calculation represented bythe following equation based on the deviation ΔS calculated in thedeviation calculating means 30 and the gain constants Kp, Ki and Kd setby the gain constant setting means 32 to provide a PID calculation valueKpid:

    Kpid=Kp×ΔS+Ki×ΣΔS+Kd×{ΔS(n-3)-.DELTA.S(n)}

Specifically, in the PID calculating means 31, the following calculatingoperations are carried out: a proportional operation for multiplying thedeviation ΔS by the gain constant Kp; an integrating operation formultiplying a sum ΣΔS of products of the deviations ΔS by the gainconstant Ki; a differentiating operation for multiplying a differencebetween the deviation ΔS(n-3) determined a predetermined time ago (e.g.,the last but two times ago) and the current deviation ΔS (n) by the gainconstant Kd; and an adding operation for adding values resulting fromthe above operations.

The PID calculating means 31 is arranged to limit the PID calculationvalue Kpid to be not more than a predetermined limit value, to limit thevalues provided by the proportional operation, the integrating operationand the differentiating operation constituting the PID calculation tomutually independent predetermined limit values, or to limit adifference between the last PID calculation value Kpid and the currentPID calculation value Kpid to a predetermined limit value.

Specifically, to limit the PID calculation value Kpid to be not morethan the predetermined limit value, the PID calculation value Kpid iscompared with the limit value. If Kpid> the limit value, the PIDcalculation value Kpid outputted from the PID calculating means 31 isequal to the limit value. If Kpid≦ the limit value, the PID calculationvalue outputted from the PID calculating means 31 is equal to Kpid.

Moreover, the limit value used in the PID calculating means 31 isdetermined by a limit value determining means 36. The limit valuedetermining means 36 is arranged to determine the limit value based onthe presumed vehicle speed determined in the reference speed determiningmeans 24, the result of the determination of the vibration by thesuspension vibration determining means 34 and the result of thedetermination of the friction coefficient of the road surface by thefriction coefficient determining means 35.

Further, the PID calculating means 31 has a low-pass filter function topermit the passage of a lower frequency component of the PID calculationvalue Kpid, or to permit the passage of a lower frequency component ofthe calculation values resulting from the proportional operation, theintegrating operation and the differentiating operation constituting thePID calculation. For example, to permit the passage of the lowerfrequency component of the PID calculation value Kpid, a filterprocessing is carried out in the PID calculating means 31 according tothe following equation:

    Kpid(n)=Kpid(n-1)+{Kpid(n-1)-Kpid (n)}/N

wherein N represents a filter degree. The passage area in the low-passfilter, i.e., the filter degree N is set by a passage area setting means37. In the passage area setting means 37, the filter degree N is set inthe form of a function of the presumed vehicle speed determined in thereference vehicle speed determining means 24 according to the followingequation:

    N=fN(VR)

An output from the PID calculating means 31 is inputted to the controlmode deciding means 33. The control mode deciding means 33 decides acontrol mode by comparing the PID calculation value Kpid inputtedthereto from the PID calculating means 31 with threshold values K₁ andK₂ on a previously established map, and applies a control value forcontrolling the operation of the braking liquid pressure regulatingmeans 3_(F) in the decided control mode to the front wheel solenoiddrive means 25_(F).

The above-described map is established as shown in FIG. 6. A pluralityof areas of a pressure-reducing mode, a maintaining mode and apressure-increasing mode determined by the threshold values K₁ and K₂varied depending upon the presumed vehicle speed determined in thereference vehicle speed determining means 24 are prepared depending uponthe result of the determination of the road surface friction coefficientby the friction coefficient determining means 35. Moreover, thethreshold values K₁ and K₂ are varied depending upon whether the wheelspeed is being decreased or increased. When the wheel speed is beingdecreased, the threshold values K₁ and K₂ are set as shown by dashedlines in FIG. 6. When the wheel speed is being increased, the thresholdvalues K₁ and K₂ are set as shown by solid lines in FIG. 6. Thus, thethreshold values K₁ and K₂ are set slightly larger when the wheel speedis being decreased, than when the wheel speed is being increased.

On the basis of this map, the pressure-reducing mode is selected whenKpid≦K₁ ; the maintaining mode is selected when K₁ <Kpid≦K₂ ; and thepressure-increasing mode is selected when K₂ <Kpid. In thepressure-reducing mode, the normally-closed solenoid valve 8_(F) iscontrolled in the braking liquid pressure regulating means 3_(F), sothat it is opened with a predetermined duty in a condition in which thenormally-opened solenoid valve 7_(F) is in its closed state. In themaintaining mode, both of the normally-opened solenoid valve 7_(F) andthe normally-closed solenoid valve 8_(F) are closed. In thepressure-increasing mode, the normally-opened solenoid valve 7_(F) iscontrolled so that it is opened with a predetermined duty in a conditionin which the normally-closed solenoid valve 8_(F) is in its closedstate.

Further, when the amount of variation in PID calculation value Kpidwithin a predetermined time exceeds a predetermined value, the controlmode deciding means 33 selects the control mode different from thatdecided by the current PID calculation value Kpid. The amount ofvariation in PID calculation value Kpid within the predetermined timemay be determined, for example, by calculating a difference ΔK betweenthe current PID calculation value Kpid(n) and the PID calculation valueKpid(n-1) determined a predetermined time ago (e.g., at the last time),or by differentiating the PID calculating value Kpid. For example, whenthe difference ΔK exceeds a predetermined positive value, the controlmode is changed in a sequence of pressure-reducing mode→maintainingmode→pressure-increasing mode. When the difference ΔK exceeds apredetermined negative value in a negative direction, the control modeis changed in a sequence of pressure-increasing mode→maintainingmode→pressure-reducing mode.

The operation of this embodiment will be described with reference toFIGS. 7 to 13. As the wheel speed, the actual vehicle speed and thetarget wheel speed are varied as shown in FIG. 7A, the deviation ΔS(=SRobj-SR) between the target slip rate SRobj set in the target sliprate setting means 29 and the slip rate SR calculated in the slip ratecalculating means 28 is varied as shown in FIG. 7B. Further, as the PIDcalculation value Kpid calculated in the PID calculating means 31 isvaried as shown in FIG. 7C, the control mode as shown in FIG. 7D isselected in the control mode deciding means 33, and in accordance withthis, the braking pressure is varied as shown in FIG. 7E. In otherwords, the control mode deciding means 33 decides the control mode bycomparing the PID calculation value Kpid calculated in the PIDcalculating means 31 with the threshold values K₁ and K₂ on thepreviously established map, and controls the operation of the brakingliquid pressure regulating means 3_(F) in the decided control mode.Thus, it is possible to control the braking pressure in a simplestructure using the normally-opened solenoid valves 7_(F) and 7_(R) andthe normally-closed solenoid valves 8_(F) and 8_(R) which areinexpensive on-off operated solenoid valves as solenoid valves eachconstituting a portion of each of the braking liquid pressure regulatingmeans 3_(F) and 3_(R), thereby preventing the wheel from being falleninto a locked state.

Moreover, in the control mode deciding means 33, a plurality of maps areprepared depending upon the result of the determination of the frictioncoefficient of the road surface by the friction coefficient determiningmeans 35 in which the threshold values K₁ and K₂ are varied inaccordance with the presumed vehicle speed determined in the referencevehicle speed determining means 24 to define the areas of the controlmodes. Thus, the control mode suitable for variations in vehicle travelspeed and road surface friction coefficient can be selected to performan effective antilock brake control.

The threshold values K₁ and K₂ during reducing of the wheel speed areset as shown by the dashed lines in FIG. 7C, whereas the thresholdvalues K₁ and K₂ during increasing of wheel speed are set slightlysmaller than that during reducing of wheel speed as shown by the solidline in FIG. 7C. By the fact that the threshold values K₁ and K₂ arechanged depending upon whether the wheel speed is being reduced orincreased, the pressure-reducing mode is selected early during reducingof the wheel speed, and the pressure-increasing mode is selected earlyduring increasing of the wheel speed. Thus, it is possible to select thecontrol mode appropriately corresponding to the behavior of the wheelspeed.

Further, in the control mode deciding means 33, when the PID calculatingvalue Kpid is varied in a positive direction to exceed a predeterminedvalue, e.g. the difference ΔK between the current PID calculation valueKpid(n) and the PID calculation value Kpid(n-1) determined apredetermined time ago (e.g., at the last time) exceeds a predeterminedpositive value, the control mode is changed in a sequence ofpressure-reducing mode→maintaining mode→pressure-increasing mode. Whenthe PID calculation value Kpid is varied in a negative direction toexceed a predetermined value, e.g., the difference ΔK exceeds apredetermined negative value in a negative direction, the control modeis changed in a sequence of pressure-increasing mode→maintainingmode→pressure-reducing mode. In this case, an excessive control or acontrol delay cannot occur. That is, when the PID calculation value Kpidis varied as shown in FIG. 8B as the wheel speed, the actual vehiclespeed and the target wheel speed are varied as shown in FIG. 8A, thedifference ΔK between the PID calculation values Kpid is provided asshown in FIG. 8C. When the difference ΔK exceeds a predeterminedpositive value, as well as when the difference ΔK exceeds apredetermined negative value in a negative direction, the control modeis changed as shown by a solid line in FIG. 8D. In other words, when theamount of variation in PID calculation value Kpid within a predeterminedtime is large even if the control mode decided by the PID calculationvalue Kpid calculated in the PID calculating means 31 is not changed,the control mode is changed. Therefore, it is possible to perform anappropriate antilock brake control free from an excessive control or acontrol delay. On the contrast, when the control mode is decided by onlythe PID calculation value Kpid, the control mode is changed as shown bya dashed line in FIG. 8D. As a result, if the control mode isestablished in serious consideration of a continuity in order tosmoothly conduct the control, the accommodation to a variation insituation of road surface is liable to be retarded. If the control modeis established in serious consideration of a responsiveness, the controlis liable to be excessive. In order to establish data to reconcile thecontinuity and the responsiveness, detailed data for deciding thecontrol mode are required, resulting in the need for a great deal oflabor.

In the PID calculation in the PID calculating means 31, the gainconstants Kp, Ki and Kd in the proportional, integrating anddifferentiating operations are each set in the gain constant settingmeans 32 in the form of the function fp(VR), fi(VR), fd(VR) of thepresumed vehicle speed determined in the reference vehicle speeddetermining means 24. Therefore, the appropriate gain constants Kp, Kiand Kd corresponding to the vehicle travel speed can be reflected to thePID calculation. Moreover, the functions fp(VR), fi(VR) and fd(VR) arechanged based on the result of the determination of the frictioncoefficient in the friction coefficient determining means 35 and theresult of the calculation of the wheel acceleration or deceleration inthe wheel acceleration or deceleration calculating means 21_(F) and21_(R). Therefore, a stable antilock brake control can be performed bycarrying out the control of the braking pressure by providing a PIDcalculation value appropriately corresponding to the frictioncoefficient of a travel road surface and the wheel acceleration ordeceleration.

In the PID calculating means 31, the PID calculation value is limitedwithin the predetermined limit value, or the calculation values providedby the proportional, integrating and differentiating operationsconstituting the PID calculation are limited to the mutually independentlimit values, respectively, or the difference between the last PIDcalculation value and the current PID calculation value is limited tothe predetermined limit value. When the PID calculation value Kpid islimited within the predetermined limit value, the inversion of the PIDcalculation value Kpid is hastened by the limit value, as shown in FIG.9A, so that the time duration T_(OUT) of the pressure-reducing mode isrelatively shortened, and the time duration T_(IN) of thepressure-increasing mode is also shortened and further, the start of thepressure-increasing mode is also hastened. On the contrast, when the PIDcalculation value Kpid is not limited within the predetermined limitvalue, the time duration T_(OUT") of the pressure-reducing mode isrelatively prolonged, as shown in FIG. 9B, and in accordance with this,the time duration T_(IN") of the pressure-increasing mode is alsorelatively prolonged, and the start of the pressure-increasing mode isalso retarded. Therefore, even if the PID calculation value Kpid is toolarge due to the generation of a noise around the calculation system orthe like, the difference between a wheel-slipping state assumed in thecalculation system and an actual wheel-slipping state can be relativelyreduced, thereby avoiding the occurrence of a delay of the brakingpressure control even if a sudden variation occurs in the wheel-slippingstate due to a variation in road surface friction coefficient or thelike, thus enhancing the control followability.

When the calculation values provided by the proportional, integratingand differentiating operations constituting the PID calculation arelimited to the independent predetermined limit values, respectively,these calculation values are limited as shown in FIGS. 10, 11 and 12,respectively. If the proportional calculation value (Kp×ΔS) is limitedby the limit value as shown in FIG. 10A, the PID calculation value Kpidis less enlarged, as compared with the case where the PID calculationvalue Kpid is enlarged when the proportional calculation value (Kp×ΔS)is not limited as shown in FIG. 10B. Thus, a possibility of an excessivecontrol is eliminated. If the calculation value (Ki×ΣΔS) provided by theintegration is limited by the limit value, as shown in FIG. 11A, theinversion of the PID calculation value Kpid is hastened, which cancontribute to an enhancement in control followability. Specifically, inthe case of the non-limited condition shown in FIG. 11B, the integrationcalculation term representing the continuity of the intrinsic control istoo large, so that the proportion thereof occupied for the PIDcalculation value Kpid is increased. As a result, when a suddenreduction or a sudden increase in wheel speed occurs due to a variationin friction coefficient of a road surface or the like, the PIDcalculation value Kpid is delayed from the state of the actual wheelspeed, bringing about a degradation in control followability. Further,if the calculation value {Kd×(ΔS(n-3)-ΔS(n)} provided by thedifferentiation is limited by the limit value, as shown in FIG. 12A, theinfluence of a chattering of the wheel speed, even if such chattering isproduced, can be avoided to perform a sufficient pressure increasing,thereby stabilizing the control. When the vehicle is traveling on a badroad, or when the vehicle has run over a protrusion on a road surface, awheel speed having a wave form caused by a vibration of the wheel or thevehicle body may be different from an actual wheel speed and inputted tothe calculation system in some cases. Thus, when the calculation valueprovided by the differentiation is not limited, as shown in FIG. 12B,the differentiation term intrinsically improving the followability isexcessively reflected to the PID calculation value Kpid, resulting in anexcessive control to bring about a degradation in riding comfort.

Further, when the difference between the last PID calculation valueKpid(n-1) and the current PID calculation value Kpid(n) is limited tothe predetermined limit value, it is possible to avoid the generation ofa large variation in PID calculation value Kpid, thereby enhancing thecontrol followability and stabilizing the control.

In the PID calculating means 31, the filter process for permitting thepassage of a lower frequency component of the PID calculation valueKpid, or the filter process permitting the passage of a lower frequencycomponent of the calculation values provided by the proportional,integrating and differentiating operations constituting the PIDcalculation is carried out in addition to the above-described limitingprocess.

A characteristic provided when the filter process permitting the passageof the lower frequency component of the PID calculation value Kpid hasbeen carried out is shown by a solid line in FIG. 13. A characteristicprovided when such filter process has not been carried out is shown by adashed line in FIG. 13. As the wheel speed, the actual vehicle speed andthe target wheel speed are varied as shown in FIG. 13A, the deviationΔS, the PID calculation value Kpid, the control mode and the brakingpressure are varied as shown in FIGS. 13B to 13E. As apparent from FIG.13, when the filter process has not been carried out, a chattering ofthe wheel speed, if it has been produced, is reflected, as it is, to thePID calculation value Kpid, resulting in an unstable control. On thecontrast, the control can be stabilized by carrying out the filterprocess to enhance the riding comfort.

Although the embodiment of the present invention has been described indetail, it will be understood that the present invention is not limitedto the above-described embodiment, and various modifications in designmay be made without departing from the spirit and scope of the inventiondefined in claims.

For example, the limit value may be set to the deviation calculated inthe deviation calculating means 30, and the present invention isapplicable not only to a motorcycle, but also to a four-wheel automobilevehicle.

What is claimed is:
 1. An antilock brake control system for a vehicle,comprising wheel speed detecting means for detecting a wheel speed, avehicle speed presuming means for determining a vehicle speed based onthe wheel speed detected by said wheel speed detecting means, a sliprate calculating means for calculating a slip rate based on the wheelspeed detected by said wheel speed detecting means and the presumedvehicle speed determined in said vehicle speed presuming means, a targetslip rate determining means for determining a target slip rate based onthe presumed vehicle speed determined in said vehicle speed presumingmeans, a deviation calculating means for calculating a deviation betweenthe target slip rate determined in said target slip rate determiningmeans and the slip rate calculated in the slip rate calculating means,and a PID calculating means for carrying out a PID calculation based onthe deviation determined in said deviation calculating means, wherein anoperation of braking liquid pressure regulating means is controlledbased on the result of a calculation in the PID calculating means, andwherein the PID calculating means is arranged to limit a PID calculationvalue to be not more than a predetermined limit value.
 2. An antilockbrake control system for a vehicle, comprising wheel speed detectingmeans for detecting a wheel speed, a vehicle speed presuming means fordetermining a vehicle speed based on the wheel speed detected by saidwheel speed detecting means, a slip rate calculating means forcalculating a slip rate based on the wheel speed detected by said wheelspeed detecting means and the presumed vehicle speed determined in saidvehicle speed presuming means, a target slip rate determining means fordetermining a target slip rate based on the presumed vehicle speeddetermined in said vehicle speed presuming means, a deviationcalculating means for calculating a deviation between the target sliprate determined in said target slip rate determining means and the sliprate calculated in the slip rate calculating means, and a PIDcalculating means for carrying out a PID calculation based on thedeviation determined in said deviation calculating means, wherein anoperation of braking liquid pressure regulating means is controlledbased on the result of a calculation in the PID calculating means, andwherein the PID calculating means is arranged to limit calculationvalues provided by a proportional operation, an integrating operationand a differentiating operation to mutually independent predeterminedlimit values, respectively.
 3. An antilock brake control system for avehicle, comprising wheel speed detecting means for detecting a wheelspeed, a vehicle speed presuming means for determining vehicle speedbased on the wheel speed detected by said wheel speed detecting means, aslip rate calculating means for calculating a slip rate based on thewheel speed detected by said wheel speed detecting means and thepresumed vehicle speed determined in said vehicle speed presuming means,a target slip rate determining means for determining a target slip ratebased on the presumed vehicle speed determined in said vehicle speedpresuming means, a deviation calculating means for calculating adeviation between the target slip rate determined in said target sliprate determining means and the slip rate calculated in the slip ratecalculating means, and a PID calculating means for carrying out a PIDcalculation based on the deviation determined in said deviationcalculating means, wherein an operation of braking liquid pressureregulating means is controlled based on the result of a calculation inthe PID calculating means, and wherein the PID calculating means isarranged to limit a difference between a last PID calculation value anda current PID calculation value to a predetermined limit value.
 4. Anantilock brake control system for a vehicle according to claim 1, 2 or3, further including a friction coefficient determining means fordetermining a friction coefficient of a road surface, a suspensionvibration determining means for determining a vibrated state of asuspension, a limit value determining means for determining said limitvalue in said PID calculating means, said limit value being determinedbased on the presumed vehicle speed determined in said vehicle speedpresuming means, a determination in said friction coefficientdetermining means, and a determination in said suspension vibrationdetermining means.
 5. An antilock brake control system for a vehicleaccording to claim 4, wherein said friction coefficient determiningmeans is arranged to determine the friction coefficient of the roadsurface by comparing a vehicle deceleration calculated based on thepresumed vehicle speed determined in said vehicle speed presuming meansand the wheel speed detected by said wheel speed detecting means, with apreset threshold value.
 6. An antilock brake control system for avehicle according to claim 4, wherein said suspension vibrationdetermining means is arranged to determine the vibrated state of thesuspension based on the presumed vehicle speed determined in saidvehicle speed presuming means and the wheel speed detected by said wheelspeed detecting means.